Tires are sometimes desired with treads for promoting traction on wet surfaces. Various rubber compositions may be proposed for tire treads.
For example, tire tread rubber compositions which contain high molecular weight, high Tg (high glass transition temperature) diene based elastomer(s) might be desired for such purpose particularly for wet traction (traction of tire treads on wet road surfaces). Such tire tread may be desired where its reinforcing filler is primarily precipitated silica which may therefore be considered as being precipitated silica rich.
When such elastomers have a high uncured rubber viscosity (e.g. Mooney, ML1+4. 100° C., viscosity), petroleum based rubber processing oil may be blended with the elastomer(s) to reduce the rubber composition's uncured viscosity and to thereby promote more desirable processing conditions for the uncured rubber composition. The petroleum based rubber processing oil can be added to the elastomer prior to its addition to an internal rubber mixer (e.g. a Banbury rubber mixer) or be added to the rubber composition in the mixer to reduce the viscosity of the rubber composition both in the internal rubber mixer and for subsequent rubber processing such as in a rubber extruder.
Here, the challenge is to reduce the cured stiffness of such tread rubber compositions, as indicated by having a lower storage modulus G′ at about −20° C., when the tread is intended to be also useful for low temperature winter conditions, particularly for vehicular snow driving.
It is considered that significant challenges are presented for providing such tire tread rubber compositions for maintaining their wet traction while promoting low temperature (e.g. cold weather) performance.
To achieve such balance of tread rubber performances, it is proposed to evaluate providing a tread rubber composition containing a high Tg (glass transition temperature) elastomer together with an elastomer having a lower Tg to beneficially promote a lower stiffness of the cured rubber composition to improve cold weather performance of the tire tread, while substantially maintaining the tire tread's wet traction capability.
For such challenge, it is proposed to evaluate providing a combination of high Tg and lower Tg styrene/butadiene elastomers together with a low Tg cis 1,4-polybutadiene rubber (PBd) with reinforcing filler comprised of precipitated silica, which may be a pre-hydrophobated precipitated silica, and rubber reinforcing carbon black.
The combination of styrene/butadiene elastomers is proposed to be comprised of a relatively high Tg organic solvent polymerization prepared styrene/butadiene elastomer (S-SBR) to promote wet traction, wherein the high Tg S-SBR is extended with triglyceride based vegetable oil to promote cold weather (winter) tire performance, together with a lower Tg aqueous emulsion polymerization prepared styrene/butadiene elastomer (E-SBR), which is not oil extended, where the E-SBR has a Tg which lower than the Tg of the S-SBR, to beneficially promote lowering the stiffness of the cured rubber composition at about −20° C. by compensating for the presence of the higher Tg S-SBR to thereby further promote cold weather (winter) performance for the rubber composition.
To meet the challenge of providing good cold weather (winter) performance while maintaining wet traction for the tire tread, it is also desired to promote beneficial processability of the uncured rubber composition which contains the high Tg S-SBR by extending the S-SBR with vegetable oil instead of petroleum based rubber processing oil. Such vegetable oil extension of the S-SBR further promotes a beneficially lower cured stiffness of the tread rubber composition at lower temperatures to thereby further promote cold weather performance for the tire tread.
Vegetable triglyceride oil extension of the high Tg S-SBR is to be distinguished from free addition of the vegetable triglyceride oil to the high Tg S-SBR or to the rubber composition. By the term “extension” it is meant that the vegetable oil is added to a cement comprised of a composite of solvent solution of the high Tg S-SBR as a product of polymerization of styrene and 1,3-butadiene monomers is an organic solvent solution with a suitable catalyst to promote the polymerization, wherein the high Tg S-SBR is recovered from the cement as a composite of the high Tg S-SBR and vegetable triglyceride oil.
The innovation of this approach thereby relies on the use of a relatively high Tg vegetable triglyceride oil extended high Tg S-SBR elastomer with a combination of with lower Tg E-SBR and low Tg PBd elastomers together with precipitated silica reinforcement which may be a pre-hydrophobated precipitated silica.
In one embodiment, the rubber composition may, if desired, also contain freely added vegetable triglyceride oil, in addition to vegetable triglyceride oil contained in the vegetable oil extended high Tg SBR, to further promote a lower rubber stiffness at lower temperatures for the tread rubber. By the term “freely added”, it is meant that the vegetable oil is added to the rubber composition containing the S-SBR during its physical mixing of rubber and rubber compounding ingredients in contrast to the aforesaid “extending” of the high Tg S-SBR itself.
In one embodiment, to promote wet traction for such evaluation, without significantly detracting from the low temperature performance, it is desired to further evaluate providing at least one traction resin in the tread rubber composition.
Historically it is recognized that triglyceride based vegetable oils such as, for example, soybean oil, has been previously suggested for addition to various rubber compositions such as for example, and not intended to be limiting, in U.S. Pat. Nos. 7,919,553, 8,100,157, 8,022,136 and U.S. Pat. No. 8,044,118.
However, while vegetable triglyceride oils have previously been mentioned for use in various rubber compositions, including rubber compositions for tire components, use of vegetable triglyceride oils as an extender oil for a high Tg S-SBR combined with a blend of lower Tg PBd and E-SBR elastomers together with precipitated silica reinforcement, which may be in a form of pre-hydrophobated precipitated silica, is believed to be novel and a significant departure from past practice to both aid in processing of the uncured rubber composition and to provide cured rubber composition for a tire tread to promote a combination of wet traction and low temperature cold weather performance.
In the description of this invention, the terms “compounded” rubber compositions and “compounds” are used to refer to rubber compositions which have been compounded, or blended, with appropriate rubber compounding ingredients. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The amounts of materials are usually expressed in parts of material per 100 parts of rubber by weight (phr).
The glass transition temperature (Tg) of the solid elastomers may be determined by DSC (differential scanning calorimetry) measurements, as would be understood and well known by one having skill in such art. The softening point of a resin, where appropriate, may be determined by ASTM E28 which might sometimes be referred to as a ring and ball softening point.